Truncation of the gp41 construct just past its transmembrane domain, leaving residues 1-194, results in a well-structured protein, soluble in detergent micelles. Although previously believed to adopt its natural homo-trimeric form, we have found strong evidence for a rapid dynamic equilibrium between monomeric and trimeric forms of the protein, which can be shifted to the mostly trimeric form by choice of suitable detergents and detergent: protein stoichiometry. The NOE data and 15N relaxation rates show a well structured helical conformation for residues 5-14 of the fusion domain, followed by a more disordered segment which connects it to the ecto-domain. There is no evidence for direct interaction with the transmembrane fusion domain in studies carried out at pH4, and both light scattering, SAXS, and NMR diffusion data point to a mass ratio of detergent:protein slightly above 1.0. The protein is found to be quite stable, even at temperatures of 40C. The fusion domain exhibits effective correlation times that are much shorter than for the ecto domain, indicating that the intact fusion domain helices are highly mobile within the detergent micelle/protein aggregate, while retaining their helical conformation. This excludes the previously hypothesized interaction with the gp41 transmembrane domain under our conditions of pH and detergent.